This invention relates to the recovery of energy from hot exhaust gases, and in particular, from exhaust gases produced by the combustion of sulfur-containing fuels, especially in the operation of a power plant.
The problem of disposing of exhaust gases from combustion produced by sulfur-containing fuel is a major concern worldwide. Due to the high corrosiveness of sulfuric acid which condenses near 280.degree. F., the heat recovery of the flue gas usually by means of an air preheater or economizer has been limited to 300.degree. F.
Furnaces using sulfur-containing fuels, especially coal, generally employ a flue gas desulfurization process (FGD) prior to discharge to the stack. Most FGD processes precool the flue gas to low temperatures, e.g., 110.degree. to 150.degree. F. by mixing the flue gas with a large amount of water in the front section of a scrubber. It is extremely difficult to recover the heat because of corrosion problems and the temperature level. Therefore a large amount of energy is wasted.
For more than a half century, stone or ceramic packed regenerators have been used for heat/cold recovery and purification in various industries, especially in coke oven gas plants for hydrogen and air separation plants. Recently several patents have been issued or applied for using such regenerators for exhaust heat recovery, i.e. U.S. Pat. No. 4,265,088 by H. Funk, and German Patent Application No. P 31 39 153.2, by G. Linde (also described in Dr. Linde's article "Improved Energy Generation from Combustion Processes as a Result of Optimum Utilization of Flue Gas Exhaust Heat", Linde Reports on Science and Technology, 34/1982, pp. 34-38). H. Funk's patent employs compression and expansion of the gas and is not readily adaptable for combustion air preheating. G. Linde's system employs regenerators for combustion air preheating to replace conventional air preheaters, but is not readily adaptable to combustion gases containing particulates since it will require either the lowering of the combustion air temperature or the operation of the particulate removal unit at a much higher temperature than normal. The reason for these alternatives is that the regenerators must be used downstream of the particulate removal equipment (i.e. electrostatic precipitator or baghouse) to avoid becoming plugged, the plugging being otherwise especially acute when coal is burned. The highest temperature achievable for the combustion air from the regenerator is thus limited by the electrostatic precipitator or baghouse which is normally operated at less than 350.degree. F., much lower than the combustion air temperature (500.degree.-600.degree. F.) normally used. To raise the combustion air temperature to such a high level requires operating the precipitator, flue gas blower and regenerator at temperatures of more than 200 degrees above normal. With conventional technology, this can be quite costly and power consuming. Conversely, there is presently being developed by Hans D. Linhardt a high temperature particle separator for particle control in coal gasification plants, details of which are described in Report No. L & A 1100-13 dated Feb. 28, 1983, "Final Technical Report--Design and Development of a High Temperature Particle Separator for Particle Control in Coal Gasification Plants" by Hans D. Linhardt, under Contract No. De-AC01-81 FEO 5120 for the U.S. Department of Energy, Division of Coal Conversion and Utilization, and also U.S. patent application, Ser. No. 345,073 dated Feb. 4, 1982, "Axis Symmetrical Separator for Separating Particulate Matter from a Fluid Carrying Medium" by Hans D. Linhardt, the government report being incorporated by reference herein, and to the extent permitted by Hans D. Linhardt, the patent application as well.
Furthermore, in the utilization of the flue gas waste heat, there is the problem of recovering the low grade of energy available due to the relatively low temperature level involved with G. Linde's process. Even with H. Linhardt's U.S. Pat. No. 4,292,050 using a Rankine Cycle to generate electricity from the waste heat, low efficiencies are usually obtained.